Cracks that form within materials can be difficult to detect and almost impossible to repair. A successful method of autonomically repairing cracks that has the potential for significantly increasing the longevity of materials has been described, for example, in U.S. Pat. No. 6,518,330 to White et al. This self-healing system includes a material containing, for example, solid particles of Grubbs catalyst and capsules containing liquid dicyclopentadiene (DCPD) embedded in an epoxy matrix. When a crack propagates through the material, it ruptures the microcapsules and releases DCPD into the crack plane. The DCPD then contacts the Grubbs catalyst, undergoes Ring Opening Metathesis Polymerization (ROMP), and cures to provide structural continuity where the crack had been.
A wide variety of autonomous self-healing materials have been developed, and these materials can include a diverse array of healing agents beyond DCPD and Grubbs catalyst. Examples of self-healing material systems include chemistry based on polydimethylsiloxane (Cho et al., Adv. Mater. 2006, 18, 997-1000; Keller et al., Adv. Funct. Mater. 2007, 17, 2399-2404; Beiermann et al., Smart Mater. Struct. 2009, 18, 085001-7; Cho et al., Adv. Mater. 2009, 21, 645-649), a tungsten-catalyzed metathesis of bicyclic monomers (Kamphaus et al., J. R. Soc. Interface 2008, 5, 95-103), and activation of latent functional groups in the polymer matrix with common organic solvents (Caruso et al., Macromolecules 2007, 40, 8830-8832) and/or with epoxy-solvent mixtures (Caruso, Adv. Funct. Mater. 2008, 18, 1898-1904). These previous studies have used single-walled, liquid containing capsules prepared either by in situ emulsion polymerization or by interfacial polymerization techniques.
High temperatures and large shear stresses are common in processing polymeric materials and composites. Structural epoxy thermosets typically are cured at temperatures between 100-200° C. Typical thermoplastic materials are extruded under high shear stresses at temperatures greater than 150° C. Preparing self-healing versions of these materials has been difficult or impossible, as the conventional capsules used for the healing agents have been unable to survive the thermal and/or mechanical demands of the processing conditions. For example, the incorporation of liquid-filled capsules into polymers to impart self-healing functionality can be problematic if the processing temperature of the polymer is near the boiling point of the encapsulated liquid or near the degradation point of the capsule shell wall.
It is desirable to provide capsules that can encapsulate a healing agent, particularly a polymerizer, within a polymer matrix that is formed at high temperatures and/or shear stresses. It is also desirable to provide a method of making such capsules that is simple and scalable.